Anechoic sonar calibration pool



June 16, 1964 c. E. GREEN ANECHOIC SONAR`CALIBRATION POOL Filed March 27, 1961 3 Sheets-Sheet l Fig. 2

IN VEN TOR. CHRL ES E. GREEN Z BY June 16, 1964 c. E. GREEN 3,137,362

vANEICHOIC SONAR CALIBRATION POOL Filed March 27, 1961 3 Sheets-Sheet 2 IN VEN TOR. CHA/n Es E. GREEN eid June 16, 1964 C. E. GREEN 3,137,362

-ANIEICHOIC SONAR CALIBRATION POOL Filed March 27, 1961 5 Sheets-Sheet 3 Fig. 7

" l CHARLES E. GREEN RIM OF POOL I- EDGE OF BOWL INVENTOR.

BOTTOM CENTER oF Poor. C /C// rw/ United States Patent States of America as represented by the Secretary of the Navy Filed Mar. 27, 1961, Ser. No. 98,719 16 Claims. (Cl. 181-5) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates in general to acoustical chambers and in particular is an anechoic sonar test pool for Calibrating acoustical and sonar apparatus in a substantially isolated acoustical environment where adverse ambient boundary surface reverberations are not present.

Ordinarily, test and calibration of acoustical equipment (such as sonar sets, for example) with a great deal of accuracy presents a diflicult problem in that the energy being received by the device being calibrated is mixed and cluttered with noise signals, false signals, and foreign signals due to various and sundry boundary surface reverberations from the container or chamber being used as the test environment. In general, it has been found that large bodies of water such as deep natural lakes are satisfactory for some test purposes, but it is also known that it is extremely difficult, if not impossible, to cornpletely control such environments to delete the unwanted inherent spurious signals and sounds therefrom. Consequently, judicious care must be employed in evaluating sonar calibration data obtained under such environmental circumstances, or the results will be of little value, if any. This, of course, requires the services of highly trained, high caliber personnel for distinguishing and sifting the pertinent and useful signal data from the impertinent, interference, and useless signal data.

In order to effect more laboratory-like conditions, a number of test tanks have been developed that are effective for many purposes and which provide acoustical test and calibration environments which in some degree approach the simulation of conditions found in an ocean of sea water of effectively infinite extent. Unfortunately, such prior art test tanks have considerable room for improvement if substantial acoustical isolation is desired within the test medium or area, due to the fact that reverberated signals of sufficient magnitude to interfere with the actual test signals usually occur and thereby complicate the evaluation thereof and the acoustical equipment being calibrated.

The subject invention overcomes most of the operational defects encountered in known prior art acoustical calibration facilities by the simple but profound expedient of providing a pool having a unique combination of shape, reflecting wall surfaces, acoustical trap locations and proiiles, water-container interface contours, and structural materials which substantially attenuate and dissipate unwanted acoustical energy once it has arrived at predetermined boundary locations. Hence, for most practical purposes, energy reverberations are substantially removed from the actual test area, with those remaining, if any, being only of negligible consequence, as will be shown subsequently.

It is, therefore, an object of this invention to provide an improved anechoic chamber.

Another object of this invention is to provide an improved test environment for testing and Calibrating acoustical, electromagnetic, and light energy radiating and receiving equipment.

A further objective of this invention is to provide a Sonar calibration test pool which reflects only a negligible amount of sonic energy from the boundaries thereof.

Another object of this invention is to provide improved means for accurately obtaining and measuring broadband acoustical beam patterns.

Another object of this invention is to provide an improved method of constructing a sonar test pool.

Still another objective of this invention is to provide a calibration tank which facilitates the testing of sonar and other transducers under predetermined simulated operational and environmental conditions.

A further objective of this invention is to provide a means whose construction substantially prevents the rendering of false and unreliable tests of sonar and other transducers due to reverberation and reflection of the energy being employed therein.

A still further objective of this invention is to provide an improved transducer calibration environment wherein the energy intended to be directed to a receiving transducer will reach same with only negligible interference, if an* from reverberations thereof.

Another object of this invention is to provide an auditorium chamber having regulated acoustics.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference characters designate like parts throughout the figures thereof and wherein:

FIG. l is a top diagrammatical view of a preferred embodiment of the anechoic sonar calibration pool constituting this invention.

FIG. 2 is an elevational View in cross-section of the subject invention showing typical surface contours, structure, land materials incorporated therein.

FIG. 3 is a theoretical diagram of the manner in which a part of the concave surface contour of the bowl portion of the subject invention is generated by means of a sector of an ellipse.

FIG. 4 is a representative bowl of the type that may be generated by revolving the ellipse of FIG. 3 about an axis.

FIG. 5 is a diagrammatical view of a cross-section of the central bowl generated by the use of the ellipse of FIG. 3 with end shelves attached and critical points located. j

FIG. 6 is a schematic view of the formation of the rim or edge of the subject pool with pertinent ellipse foci and representative energy rays incorporated therein.

FIG. 7 is plan View of another preferred embodiment of the invention showing somewhat pointed ends.

FIG. 8 is a representative acoustical energy pattern used in determining the anechoic properties of the subject invention.

FIG. -9 shows a plurality of oscilloscope images of signals received in the subject invention when the acoustical energy pattern of FIG. 8 is broadcast at three different angles therein.

Referring now to FIG. 1, there is illustrated an elongated or oblong embodiment 21 of the anechoic pool constituting this invention as having a circular shaped bowl 22, a shelf-like acoustical energy trap 23, a rim 24 which is partially straight edge in shape and partially semicircular in shape as viewed from the top. Bowl 22 and shelf 23 contain any preferred environmental medium which is suitable for the propagation of sonic, light, heat, or electromagnetic energy, or the like therethrough. Such environmental medium may, for example, consist of liquid or gaseous uids such as fresh water, sea water, air, or other `comparable elements. In the herein disclosed embodiment, however, bowl 22 and shelf 23 contain a pool of water adapted for having the equipment or a portion thereof submerged therein for test or calibration purposes. Y

A pair of sonar transducers Y26 and 27 are disposed` within said water at or near the center of bowl 22., One

of said transducers is ordinarily considered to be a trans-'i mitting transducer andthe other a receiving transducer transmitters and receivers orother test instrumentation or apparatus; Obviously, any number of transducers may inasmuch as Ythey are V'respectively connected to sonarV be employed to meet test requirements, and'their exacty Vor rubber compounds,V or Vanyrother material which will.;V Y absorb sound, heat, light,j electromagnetic energy, or theV like, depending on theuse to which the. subject'chamber f In addition, vother equipment and instrumentation pertinent to operation of the subject pool and apparatus'to be calibrated may be appropriately housed on said bridge means if desired. Y, l .f FIG.,2 shows a detailed cro'ss-sectionof anechoicsonar test pool21 as being mounted on any appropriate supa box or container structure having sufcient strength characteristics to properly hold the weightand .pressures of the pool. A layer of reinforced concreteV forms a lower retaining basin 29'and is of the order of'six inches thick. A layer of tar ofthe order ofgtwo inches thick is disposed ongtop of lower retaining basin 29 to form an intermediate shock-absorbing, dynamiccushion ,type

. support basin 30, which provides increased pool stability and structural strength, due to the reduced influence Vof ambient external geophysicalor atmospheric conditions, such Vas temperature andV pressure changes, vibra-V tions, or the like. Ofv course, tar basin 30 also acts las a Vcompliant layer that is capable Vof, cold flow which will iill and seal any cracks Vthat ,may inadvertentlyformlin Y generate Tim Suffa 37. .F3 S falsropl'ocaved V0.11 aXS Y* v port means 28, which may, for example, be the earth or bowl surface 35, al shelf surface36, and a curved rim surface` 37, each of which has its Vown uniqueform and location; Bowl 22 occupies the center Vportion of the pool andis lthe place where transducers 26 and. 27 are located.

The peripheryolr edge V,of said bowll contains a lip 3.8 Y

which extendsgdownwardly from bowl. surface to a substantially horizontal flat 39 which is'lower inheight Y Y than said bowl periphery. The outermostvcontinuation of at 39 becomes rim surface 37. YDisposed on Vtop of 1 flat 39`between lip 38 vand rim surface 37 is a layer -of energy absorbent.material l39a which may, for. instance,v

becomposed of mud, concrete wedges, acoustic scatterers,

4is being put. f Y Y Water 2,5,V of course,*substantially'fillsvthe entireY pool. 'o Y Y As'cursorily 'shownfin FIG. 2,'and theoretically eX- n plained in considerable detail in conjunction"V with FIGS.VK 1

vE 'through 6, any ellipse V4 0 :having foci E1 and F2 is l used to' generate a segment ofbowl surface 35.7'vv Focus A1' is, positioned ,onthe center axis of bowl 22 and f focus F2V is positioned above'V the surface Vof waterZSV adistance thatis .half as great as t-he distance AF1,` is below thefsurfaceV of said water; "However, because Focusfl-TZvis not actually limited in its disposition to av A position above the water' surface-adistance vthat is half as great as 'the distance F1 'is-below thesurface thereof,

it may alsobe placed at a'distance above the/surface of 30 l mud or otherenergyfabsorbent material is located below 1 the surface of they water, or placed at a point located contiguously with said top surface. Horizontal revolu-VVV .tion of ellipse 40 about axisY Z.Z then generates Vthe elliptical segment Aconstituting bowlj'surface 35. Like-'lk the lconcrete of the associated concrete basins.V Floating on top of tar basin 30 is anrupper reinforced concrete', basin31 which may bke-of the order of Vtwo inches thick and which may also be lined at its upper surface withany pertinentY energy absorptionand reflection material Vdesired. Although'rconcrete is disclosed herein as thejpreferred embodiment material vfor ilPPrbasin 31', it should be understood that any suitable pmaterialwhich'contains the proper operational and strength characteristics andV is susceptible to the required contour shaping maybe used. Hence such materials as plaster, metal, plastic,

berglassfplasterof Paris, orvany appropriate sounding absorbing, transmitting orreiiecting materials, Vorthe like,

f low the water surface may be Vcombined witheach other, with steel, or .other Y reinforcing materials andmaybeemployed as convenient.; Likewise, the bottom basinv mayjbecomposed Ofjsuch Y' materials, tooif found to be more expedient than reinforced concrete. MOreOVer, suitable equivalent` resilient materials maybe substituted for tar as the material fof sidered as teaching the use. thereof. i

l intermediate'basin 30, and this disclosure, should be Vcon- Each ofthe aforementionedlower, intermediate, -andfA upper basins is shown to have peripheralretaining walls 32, 33 andV 34, respectively, whichare Vsubstantially verl tical. However, said walls may be dish-shaped 'or otherwise formed to fit any environmental condition ,and still be strong enough tosupport the entire spool structure in Y itspredetermined geometricallconiiguration. One eXcep-f tion` to this is the` inside surfaces ofthe upper-'basinl These surfaces must be constructed in such manner asrtoy provide particularY and unique contours,`which areV prop-V erl'ypinterrelated to effect theV variousabsorptive and re-` flective objectives lof the invention. The explanation'of the construction of saidgcontouredV surfaces will be' presented in structural detail` below and in theorysubsef quently. n

Upper'basin 31 may lbeV divided into three f ver/y a portant surfacefsections., These-sections includeacurved 37. T he foci'of ellipse'42 are located at Fldisposed beL 1 f Y .and F5 which is Vdisposed. above the surfacel of the'watera distance equal'to halfthe distance n F1 ,isV disposed ,belowr the; surface of said water.V 0r,

again in the'alternative,fF5 may, instead, beclisposed a' V` Y distance above the surfaceof thewater equal-to the .depth Y the mud orotherY energy absorbent ma the-waterr thatis equal to the depth the topgsurfaceofthe Wise; kan ellipse 41 having .fociF3 and 'F4 is usedto tance above the surface ofthe Vwater thatth'e top surface of the energy absorbing shelf material, orfa kpoint cron- V lV tiguous therewith, is located `below'the lsurfacepofjsaid@` water.

Y -Ifso desired, another ellipse-V412 Vmay employed'tol'i-Vr` Y generate a rim surface that is ,identical with Vrim surface the top surface of Y terialgris: located `below 'the surface of'A the 'water or, flike`` w1se,.atr a place contiguousfwithsaid top surface.

The'theory of operation' of the subject inventionisfas f.

follows The transducersKorinther-equipment'usedin the Calibra# v Vtion process` areappropriately positioned .at apredeterr mined pool depth at,near, vor"aroundLcenter aXisZ.--Z

as desired to provide any givendirect'travel path between Vthe transmitte'nand receiver'transducerV elements.v` Al-"f though any preferred arrangementwillifunction well Vfor many vpractical purposes,,optimum` etlic'iency, as farjas i greverberation interference isV concerned, rostensivelyocr curs when the transmit `and receive transducers arespacedV` close together relativeto Vtheoverall size of Vthe entire pool Y Y structure: and nearthe aforesaid axis` Moreovenin' 'l eventV that, 'said vtransducers have. some directional char-yV acteris'tics lrather thanbeing omnidirectionakthe axis of .l maximum'power tor anticipated maximumr` power if the' ,i c

radiation patternY thereof'is unknown) should'be aligned withfthe'majorariis'ofthe pool structure 'so that thera'dig ationpattern of the'V transmitted energy" is.Y directed for. the l most part toward that part of the pool structure containing the largest shelf area, in order to provide optimum reverberation nullification. Hence, it can also be seen that the design length of that portion of said shelf which is in alignment with the maximum power portion of the energy radiation pattern, is a matter of choice of the artisan and is contingent upon and, in the final mali/sis, determined by the particular type of equipment to be tested. It has, therefore, been found that, the greater the radiation power in any particular direction, the longer the shelf must be in that direction in order to attenuate and trap that spurious energy which would otherwise be reflected back into the test area. anechoic pool structure should be made sufiiciently large to take care of all anticipated high power equipment to be tested, since such is the design criterion or limiting factor, and inasmuch as the calibration of equipment of lesser power would not be adversely affected by having a pool that is too large, so to speak.

' Assuming for the purpose of theoretical discussion that acoustical energy, electromagnetic energy, and light energy travels in rays and that in the preferred embodiment and an acoustical energy ray R1 travels from a sound source located at one of the foci of a plane ellipse such as F1 of FIG. 3. If a physical surface S1 is generated by rotation of said ellipse about an axis of revolution Z-Z passing through focus F1, the other of focus F2 thereof will form a circle within a plane and said generated surface forms a portion of a bowl. Now, if focus F11. is located below the surface of a pool of water at twice the distance that focus F2 is located above the surface of said water, then any sound such as that represented by R1 originating at focus F1 will be reiiected from said generated elliptical surface and tend to pass out of the water and through focus F2, as is illustrated in FIG. 3, or be reflected by the surface of the water toward the virtual image of focus F2.

1f, for instance, the generation of said elliptical surface is discontinued or terminated near axis Z-Z and continued as a gradual progressive projecting surface which contacts said Z-Z axis at a right angle, a bowl of the type essentially shown in FIG. 4 is obtained. This bowl is unique in that most of the energy radiated from an omnidirectional source strikes the concave surface thereof and reflects therebetween and the water surface toward the circumference of the circle circumscribed by focus F2, and only a negligible amount of energy is not so reflected. Of course, it should be understood that it is unnecessary to discontinue said elliptical section in the vicinity of the aforesaid Z-Z azis, and, in some instances, it may even be preferred to allow portions of said revolved elliptical section to converge to form a surface apex on said axis, rather than being discontinued to form a projecting surface normal thereto.

If the" aforementioned bowl surface obtained by generation of said ellipse of revolution is discontinued in such manner that the periphery thereof has a radius that is shorter than the radial distance between axis Z-Z and focus FZ, and if a shelf extension is attached to said periphery, as is illustrated in FIG. 5, the aforesaid reected energy is reiiected onto the upper surface of said shelf, Where it may be partially absorbed by the previously disclosed absorbent trap material at F2v, the virtual image of F2.

Consider now the various relative parameters. For the sake of using dimensionless units, the operational depth of the transducers will be given as unity and all other dimensions will be referred to this distance due to the relativity thereof therewith.

One of the critical parameters involved is the depth of the pool (Dp). If the bottom were a perfect reector, F1 might be located at a half-depth position; however, where the bottom is absorptive, the effective depth actually falls below the real depth, and further concerns a problem of acoustic loading. But, since the actual or Obviously, the entire effective depth is a controllable parameter, no insurmountable problem occurs.

Another parameter is the transducer depth (Dt), which is usually found to be optimum at half of the pool depth.

Another controllable parameter is the depth below the water surface of the shelf area (Ds), where, for the most part, energy absorption takes place. It has been found that these three parameters are crictically related and an important factor in determining bowl diameter. Of course, other factors such as volume of water, distance separating the transducers, test frequency range and test power being used must also be considered in determining actual overall size and dimensions, but inasmuch as relative dimensions appear to be more critical from an invention standpoint, it will be left to the design skill of the artisan to decide how big or small the overall test pool should be made, and only the relative dimensions Will herein be provided.

The following table shows a number of preferred relationships of the aforementioned parameters. All dimensions given, as previously mentioned, are related to the operational transducer depth with same being unity or 1.0.

Radius of Bowl as a Function of Pool and Shelf Depths Pool Depth (Dp):

Shelf Depth (Ds) For example, a pool depth of 1.6 means the depth of Water is 1.6 times the distance from the surface of the water to the transducers, and a shelf depth of .5 means it is half as deep as the transducers. The corresponding gures in the table represent the horizontal radius of the pool for respective pool and shelf depth, with said radius being measured from the Z-Z axis to the rim of the bowl.

Because the surface of the Water is above the top level of the bowl, the container periphery or rim must be considered. Again, the theoretical application of an ellipse of revolution about axis Z-Z is used with this ellipse having a greater distance between foci than the previously mentioned ellipse. This second ellipse is actually a variable size ellipse with the distance between foci likewise being Variable to accommodate any pattern necessary to provide any desired geometrical configuration and concomitantly taking care of reverberation attenuation. Thus, if it is desired to make the pool substantially oblong in shape, having a longitudinal major axis in one direction which is longer than a minor axis normal thereto, the distance between foci must be varied accordingly as the ellipse is revolved about axis Z-Z. The factorto be considered in this instance, of course, is shelf distance or area in any particular direction as dictated by the directional characteristics `of the transducers being tested. Obviously, any structural or geometrical configuration may be used, but for all practical purposes, it has been found that the greater the shelf distance or area, the greater the energy attenuation. Thus, if a given transducer has directional charcteristics, it should be understood that it should be mounted for energy transmission along the longitudinal major axis of an oblong pool, if possible, because reverberationattenuation would be effected to a greater extent in that direction. On the other hand, if the transducers being tested are omnidirectional, perhaps a circular pool with uniform shelf distanceor area would be more desirable. Again, as long as the herien disclosed parameter relationships and surface contoursare substantially maintained, size and geometrical configuration may be a matter of preferred selection, and reverberation attenuation will be effected sufficiently to provide a substantially anechoic test pool.

Now, as shown in FIG. 6, anotherV surface isgenerated from a portion of said second mentioned ellipse. VThis surface is the rim, and the portion of the ellipse selected to generateit should be that *whichv will effectively re'- fleet any sonic energy contacting same, which originated;Y

at one'of the transducersor its virtual image; toward the vitual images of said ellipses, which, of course, are dis- 'Y For the sake of clarposed in` the absorbent' shelf area. ity, two representative rays'are shown as'emanating from the vicinity of F1,'one striking the concave surface of said outer rim, and being reflected or, re-reect'ed toward the virtualV image ofy one of the foci of said second mentioned ellipse, and the other reflecting from thefwater surface prior to reflections from the outer rim.V Depending on the length ofthe shelf, the sound energy may bounce back and forth between the surface of the waterand the abn sorbent shelf either Vbefore'or after reiiection by said outer rim surface, land so Vdoing causes it to be attf'emtat'edV or ,beattodeath, so to speak, since each reflection andY each bit of travel reduces its energy level until it' is prac-1 tically zero. Again, it should be understood `that the eiiciency of the absorption shelf is contingent on its size transmission Vtherebetween and, consequently, the cali, bration thereof. Y 'Y 'Y i ,At lower frequencies, the geometrical-configuration of thesubjectpool, as viewed from the top, may become somewhat critical, in that Vthe outermostV wall may becorne a reflector where the Ycross-section,of the zellipitical sector isa fractionvof a'wavelength. To reduce such low frequency reflections or reverberationsnto substan- Y'tially a negligible quantity, the semi-circular fends may be replaced with ends that have beenreshaped to beVv more pointed, as'depicted diagrammatically in, VFIG.- 7.

Thisy contour tends to scatter the energy 'more and, for most practical purposes, eliminates Vthe low frequency Y end reflection problem, and the spreading loss due to and the absorption materialused. The more perfect the absorbent shelf material, the less shelf area required, and, Y

theoretically, if'a perfect absorber can be used, then ya minimum shelf at will only need to extend horizontally half thedistance of the depth of the transducer. @therwise, a longer shelf is necessary; that is, Vone ,that is large l enough to encompass all virtualimages.Y

Where two transducers are used, the preferential energy propagation direction is established and the Vshelf eX- tended further in this direction, thereby forming Van elongated pool. Although for many purposes the transducersV may be located ,at substantially the center of the pool (F1), sometimes it is necessary to separate them in order toY perform the desired calibration.

theshallow'shelf for the saine elliptical curvature ,as the transmititng transducer is moved away fromtheV center of the bowl, it may be'necessary to compensate for this fact, by further elongating the shelf.

Briey, the actual operation of the anechoicfsonar test pool is essentially as follows; f Y y lA transmitting transducer and receiving transducer are positioned as desired in the subjectrtank. As 'the transmitting transducer broadcastsits sonic energy, itfisV conducted to the receiving transducer 'by means ofthe water or other fluid medium inthe pool. Thatenergy which travels'in a direct pathl toward the receiving transducer provides the calibration I signals, WhileV that.V energyV not received directly by the "receiving transducer' travels through the water to either ,the surface Y,of the water, theV surface of the bowl, or the 'sur-face. ofthe Aouter rim.;

Upon reaching anyone of these surfaces, `said`energy is Y Y reflected, yand then perhaps re-.retlected by another @of said surfaces, and so on, until lit is trapped at substan- Y Because the virtual y image (F411) of the reected energy .moves furtherrinto Y distance and partial absorption is sufficient to permit valid response test runs between'closernounted transducers.

Otherwise, this embodiment voperates essentially like the embodiment of the subject invention having semi-circular ends: y

transducer.

radiation pattern Vof FIG. y8 for an illustration of an actual test krun in 'the subject invention andoscilloscope pictures of the results thereof. The' zero degree signal image ,of FIG. 902); isnthe pulse waveform received by the receiv,`Vr

ing transducer when the transmitting transducer is broadcasting powergdirectly thereat or at zero degrees, and jA for our purposes here is considered to bethe reference; image.y DueroY the fact ythat thetransmitting and receiving transducersv are closer tol each other than the receiving transducerfis'to theperiphery of the pool, reverberations of transmitted sonic energy, if any, wouldV appear'ini this Y image at a vtime subsequent to initial pulse 'n Atrthe zero y V,

degree 'transducer position'it canV be seen'thatilittlefor Vno reilectedenergy'has beenrerceived at the receiving 1 transducer, indicating that,l for all practical' purposes,

Y 45 the pool is truly anechoicwhen used Vin this manner.. 'Y

' lfV the ,transmitting `transducer Vis rotated softhatfthe twenty-two degree lobe of 'the energy. radiationV .pattern y' of FIG. 8 is inline with'V thefr'eceivingtransducer and tially the virtualirnage position of the vrim formingjellipse located in the absorbentV material disposedat the shelf. Assuming, for instance, that transducer26 is the transinitting transducer and'zthat itis broadcasting'ggacoustical energy toward receiving transducer 27 in; the anechoic the level thereofis increased approximately 2O dbfor` Y sufficiently to compare with the'fpoyverV ofthe zerodegree reference lobe of FIG. 9(a), Vandfextrennelyminute quan-jf f tity ofreflected` energy Y43a is received by-'the receiving 'transducer-a'short time afterl vthe initial transn'u'ssionV V,pulseras is shownin FIG.IV9( b).k Y A gllf the transmitting transducer isg'again! rotated so that i' .the one hundredgand eighty degreelobe orV positionv of i the ,energy radiation patternof'FIGyS :isfin line ,with` the receiving transducer'andr-.the level Vthereof increased 'f approximately 40 fdb or Vv.suiicient yto Vbe" comparableiwith pool of FIG. 2. Then any sonic Ypressure rays which are ,7.

not in the direct path of receiving transducer 27, con-v Y tinuein their respective travels untily they are. reflected into the' sound trap at the Vpool shelf. Upon reachingV the sonic trap, said rays are either absorbed'by the absorbent material contained therein or ,reflected back and forth between the sound trapandY the surface of the water until attenuated in the 'immedate vicinityithereof. ir'lhis atten-- uation effect occurs because each of said'-reflc'ectionsreduce the energy level ofthe sonic energy until( it is practically dissipated in the water in lthat area or in thentrap, thereby eliminating most of V.thespurious energy thatv might otherwise Vreverberate back to both the transmitting and receiving transducers and interfererwith the signal `thelevel of the zero degree reference Vlobeiof FIGQQM'), 'Y only "a vsmall amountrof rellectedenergy l44 is received by thereceiving transducer aV short period oftime after thevinitial 'transmissionfpulse, as is' sh'oWnfinFIGi ,97(c7).;3 v 1 `In,analyzingtheiwaveforrns of FIGS. 9(1a), 9(b)`,V aud:- I -9(c)., it would appear that no reverberation's'occur Vatfthev t reference zero degree'loberfp'osition, smallbutV inconsef quential reverberations occur atjthe twenty-two` degree positioni asia resultof Vreflections fromjthegedge of Athe Y bowlV andperhaps the ofthe' pool, and only anegligi- 70 bly effective Y quantity of reverberations Yoccurr at'the one Vhundred-and eighty degree 'position as a resultoffr'eecV tions from the center k'of theV bottom of the` pooh/the .v edge of the bowl, and the rim of the pool.VV These spurious reverberations -are toosmall in number'and amplitude,

however,to have any adverse effect" on;thereceived sig- L nal or the data derived therefrom during the sonar equipment calibration process. Even the layman can easily distinguish between real test signals and reverberations without difiiculty, which obviously facilitates analysis of calibration data thereby or by trained scientific personnel as Well. Moreover, the resulting data obtained from use of this invention during calibration of sonar devices are much more accurate and of considerably greater value than that ordinarily obtained in natural bodies of Water or prior art test tanks.

Although, the foregoing prepared embodiments relate to an anechoic sonar test pool wherein the energy employed for calibration purposes is acoustical in nature, it should be understood that incorporating a few well known design changes would facilitate its being used as a chamber for testing electromagnetic equipment. For instance, if the aqueous fluid or water were replaced with air, another fluid, or a medium representing atmospheric or space environmental conditions, electromagnetic energy could be propagated therein and directed by the chamber shape to provide a substantially isolated test tank. Furthermore, a similar isolated test chamber would be effected to a considerable degree without the presence of an aqueous medium such as water, even though sonic or pressure energy were being employed in the calibration process, due to the disclosed structural surface forms and materials thereof. Thus, the acoustics thereof may be so controlled as to provide an improved auditorium Where human listeners may receive the full benefit of the sound being transmitted from any given source properly located within the structure without the adverse effects of excessive echoes, reverberations, reflections, beats, and the distorted sounds produced thereby. Likewise, if so desired, energy having other appropriate wavelengths such as, for example, light may be used in conjunction with the chamber constituting this invention with equally advantageous results, if the surfaces involved were made sufficiently reflective and absorptive, respectively. It is, therefore, to be understood that although the preferred embodiments disclosed herein represent an anechoic test chamber for underwater test and calibration of sonar apparatus, that it is not t be so limited, inasmuch as the subject structure is also an anechoic chamber to other forms of energy as well and may be easily adapted therefor by one skilled in the art from the teachings herein provided.

Obviously, many modifications and variations of the present invention are possible in the light of the above disclosure. It is, therefore, to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

l. An anechoic chamber having a substantially pseudo infinite field for vibrational energy waves comprising in combination, a bowl having a concave curvilinear surface, a shelf attached to the peripheral edge of said bowl, and a rim having a concave curvilinear surface mounted on said shelf at the outer edge thereof.

2. An anechoic sonar calibration pool comprising in combination, a circular bowl, said bowl having a concave curvilinear surface, a flat shelf attached to the peripheral edge of said bowl, a rim mounted on the outer periphery of said shelf, said rim likewise having a concave curvilinear surface, and water disposed Within the limits of said outer rim in suiiicient quantity to completely iiood said bowl and said shelf.

3. An anechoic chamber comprising in combination, a circular bowl, said bowl having a concave surface defined by a first elliptical sector being rotated about an axis of revolution, a shelf attached to the outer periphery of the concave surface defined by said first elliptical sector, and a rim having a concave surface generated by rotating a second elliptical sector about said axis of revolution mounted on said shelf at the outer edge thereof.

4. An anechoic chamber comprising in combination,

a circular bowl having a concave-curved surface portion of which is generated by rotating a first ellipse about an axis of revolution passing through one focus thereof with the other focus thereof located above a predetermined plane a distance which is half the distance said one focus is located below said predetermined plane, a flat shelf substantially paralleling said predetermined plane attached to the outer periphery of said circular bowl, and a rim having a concave-curved surface mounted on said shelf at the outer edge thereof, said concave-curved surface having a portion thereof generated by rotating a second ellipse about said axis of revolution with one of the foci thereof coincident therewith and the other of the foci thereof passing through a second plane that is parallel to said predetermined plane and passes through said one of said foci.

5. An anechoic chamber comprising in combination, a bowl, said bowl having a surface profile which is defined by a sector of a first ellipse, a shelf attached to the outer edge of said bowl, said shelf having a predetermined flat surface area with all portions thereof facing toward the same direction faced by the concave-curved surface portion of said bowl and with all portions thereof lying in the same plane, and a rim attached to the periphery of said shelf having a surface sector that is elliptically curved in such manner that any energy received thereby will be effectively refiected toward the virtual image thereof or out of the chamber.

6. A pseudo-infinite field adapted for use in calibrating sonar apparatus comprising in combination, a bowl, said bowl having a circular periphery and a surface contour defined by an elliptical sector, a shelf attached to the circular peripheryof said bowl in the plane thereof, a rim mounted on the outer edge of said shelf, a pool of aqueous uid confined by the boundary of said rim of sufiicient depth to entirely fill said bowl and flood said shelf, said rim having a surface profile that is elliptically curved in said manner that acoustical energy received thereby is effectively reflected toward the virtual image of one of the foci thereof, and means attached to said bowl, said shelf, and said rim for supporting same in the aforesaid relationship.

7. The device of claim 6 wherein said bowl, said shelf, and said rim are concrete.

8. The device of claim 6 wherein said bowl, said shelf, and said rim are plastic.

9. The device of claim 6 wherein said bowl, said shelf, and said rim are fiberglass.

l0. The device of claim 6 wherein said bowl, said shelf, and said rim are of energy absorbing material.

l1. The device of claim 6 wherein said pool of aqueous fluid is water.

12. The device of claim 6 wherein said means attached to said bowl, said shelf, and said rim for supporting same include a concrete basin, and a tar basin interposed therebetween.

13. An anechoic sonar calibration pool comprising in combination, a bowl, said bowl having a circular periphery and a surface contour generated by rotation of an elliptical sector about an axis of revolution passing through the center of said circular periphery, a lip integrally connected to and extending downwardly from said circular periphery, a shelf attached to said lip, said shelf having a flat surface lying in a plane disposed between the circular periphery and bottom of said bowl and normal to said axis of revolution, a rim mounted on the outer edge of said shelf, a layer of sound absorbing material disposed on said shelf between said lip and said rim with the surface thereof substantially parallel to the surface of said shelf and flush with the extremity of said lip, said rim having a surface profile that is elliptically curved in such manner that any sound received thereby is reflected toward a focus thereof, a pool of water confined by the boundary of said rim for flooding said bowl` and said layer of sound absorbing'material at la depth sucientV for the surface thereof to re-reflect the/sound reilected'by said'ellipticallycurved'rim surface toward a" virtual image of the aforesaid focus located kwithinv said f sound absorbing material, and means attached to said bowLsaid shelf, and said rim'for supporting same in the Vaforesaid structural relationship.`

' tion passingthrough one focus thereof with the'other focus thereof disposed above a predeterminediplane adistance which islhalf `that said one" focus'lis located' belowsaid predetermined plane, a vibrational energy .wave trap 14. The device of claim 13 wherein said layer of sound absorbing material Vdisposed on said shelf'between said lip and said rirn with the surfacethereof substantially parallel to the surface of said shelf and flush With the extremity of said lip is mud. Y f

15. An anechoic chamberrada'pted for being used in Calibrating sonar equipment comprising in combination,

vhaving a concave s'urfacesportion which is generatedbyV rotating a first. elliptical sector about an axis of revoluhaving -a at shelf substantially paralleling said predetermined plane attachedto the outer'periphe'sryv of said cir-L *cular bowl, a rim surface having a'c'oncave prole eXtending across *said predetermined plane'which isgenerated by rotating a second elliptical sector about said axis of v revolution with one focus thereof coincidenttherewith and o, the other focus thereof disposed in ,aV second plane that' is parallel to `said predetermined plane, Va medium for propagating vibrational energy wave/motion connedrby the boundary of said rim surface in such manner as to completely saidbowl `and at least flood said VVtrap toV a depth where thesurface' 'thereof coincideswithsaid pre-x determinedplandand means; attached to. said bowl,` said surface profile that is elliptically curved in such manner that pressure energy received thereby is reflected toward, 'Y a'focus thereof or effectively re-reflected toward a virtual image of the aforesaid focus located within said trap means, and means connectedrto each of the aforementioned means for supporting same in the aforesaid combined relationship.

16. An anechoic chamber comprising a circular bowl trap, and said rim surface for supporting same in the-- aforementioned*structural and geometrical c'oniiguration.V

1 ,f'vRelerencesv l(ite'dinV the lerof this patent if UNITEDSTATES PATENTS 1,726,499 Noriho 'Aug;'27, 1929 1,857,641 Johnson 'May;1o,1932

2,503,400 Mason V K V.v v Apr. 11, 1950 

2. AN ANECHOIC SONAR ACLIBRATION POOL COMPRISING IN COMBINATION, A CIRCULAR BOWL, SAID BOWL HAVING A CONCAVE CURVILINEAR SURFACE, A FLAT SHELF ATTACHED TO THE PERIPHERAL EDGE OF SAID BOWL, A RIM MOUNTED ON THE OUTER PERIPHERY OF SAID SHELF, SAID RIM LIKEWISE HAVING A CONCAVE CURVILINEAR SURFACE, AND WATER DISPOSED WITHIN THE LIMITS OF SAID OUTER RIM IN SUFFICIENT QUANTITY TO COMPLETELY FLOOD SAID BOWL AND SAID SHELF. 